Unlike the construction of a conventional planographic-plate material in which the surface of an aluminum or zinc sheet is made rough by surface graining, the construction of the plate material of the present invention is characterized in that the surface of a thin sheet as a substrate is made rough by adhering finely divided, solid, metallic or nonmetallic particles to a thin sheet previously coated with an adhesive by uniform sprinkling, that is, the substrate and the finely divided solid particles function respectively as a support and a surface-roughing material.
In the plate material of the present invention, a rough surface which consists of fine spherical surfaces of finely divided solid, metallic or nonmetallic particles has the ability to form a more strongly hydrophilic water-film, as a result of desensitization of the surface with a treating solution comprising weak acids. If the adhesion of finely divided solid particles to a substrate is done under a predetermined condition of the ratio of the thickness of the adhesive layer to the average diameter of the fine solid particles, then the fine-particle layer ranging from a monoparticulate layer to a multi-particulate layer can be formed. As regards the microscopic state of the rough surface, each of the finely divided particles is adhered at its bottom to the substrate, and adjoining, finely divided particles are adhered at their respective adjoining portions to one another to form an indefinite number of extremely small voids between the adjoining, finely divided particles. Therefore, there is an advantage that the desensitizing agent penetrates deep into an indefinite number of the extremely small voids between the fine particles and thereby sufficient desensitization is achieved. By virtue of this advantage, the plate material substantially scarcely causes any scumming in printing and, hence, it is useful as an excellent plate material. In the plate material of the invention, the rough surface which has been treated with the desensitizing agent is further coated with a positive- or negative-type photosensitizing solution. Consequently, the plate material possesses an advantage that, instead of normal pre-sensitized plates, it can be put, at a relatively low price, into practical use as a plate material for offset printing.
Other features which are considered characteristic of the invention are set forth in the appended claims.
Although the invention is illustrated and described in relationship to specific embodiments, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.